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El Mono chert: A shallow-water chert from
the Pliocene Infierno Formation,
Baja California Sur, México

Jorge Ledesma-Vázquez
Facultad de Ciencias Marinas,
UABC, Ensenada B.C. México

Wednesday, July 31, 2002
3:00 pm - Pacifc Forum Room

A well bedded, 14-m-thick chert and cherty breccias unit, assigned to the Infierno Formation, is located near the southeast corner of Bahía Concepción at the base of the Concepción Peninsula. Previously unknown, the age of this chert is restricted by fossils that date conformably overlying limestones as late Pliocene in age. The chert beds, as well as the rest of the associated sequence, were deposited in an interconnected set of small basins flooded from the direction of (and very much similar to) present-day Bahía Concepción. A shallow-water environment for the chert is indicated by the occurrence of fossil mangrove roots (Avicennia), preserved in life position along the margin of one basin, and the abundant presence of the ichnofossil, Ophiomorpha, in a laterally extensive horizon within the chert body. Stratigraphic bracketing of the chert by an underlying, subaerial fan-deposit conglomerate and an overlying, fossiliferous calcarenitic limestone made mostly of coralline red algae fragments confirms a likely water depth between intertidal and 10 meters, comparable to the Isla Requesón embayment. X-ray diffraction and microscopic analysis of the chert suggest that a portion of the silica in the chert was deposited originally as a particularly silica-rich tuff. Northwest-southeast basin-cutting faults acted as conduits, transporting silica-enriched hot water that eventually transformed most of the original coralline red algae carbonates and the volcanic glass to opal-A and low cristobalite. Coastal hot springs at Santispac and at Isla Requesón may serve as a contemporary model for such geothermal activity. The northwest-southeast fault system is responsible for the outline of both Bahía Concepción and Peninsula Concepción, with the Concepción Escarpment to the east of Peninsula Concepción showing a very steep change in water depth from the west. This escarpment is clearly tectonic in origin and could exhibit a detachment between the local and regional basement.

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